There is a great variety of optical inspection systems having a common goal for locating defects existing on the patterned surface of an inspected object. The term "patterned surface" signifies such a surface which is formed with regions having different optical properties in respect of an incident radiation.
An inspection system of the kind specified typically comprises means for illuminating an object to be inspected, acquiring images formed by light reflected from the illuminated object, and image processing. However, if the inspected object is a photomask, flexible printed circuit board (PCB) or the like, whose patterned surface typically comprises transparent and opaque regions, the acquired images formed of light reflected from the illuminated surface are not indicative of such `defects` as foreign particles, for example, of dirt or dust, which may occasionally be located in the transparent regions. Indeed, it is known that a surface of such particle is not mirror like, and, accordingly, light returned from the particle is irregularly reflected, scattered light. The problem is very essential when using the photomask as a phototool in PCB, graphic arts and printing industries.
There have recently been developed methods and systems wherein the inspection is performed by means of illuminating an object and acquiring and processing images formed of reflected and transmitted beams of light. Such systems are disclosed, for example, in U.S. Pat. Nos. 5,572,598 and 5,563,702. The systems in both patents employ a so-called `scanning technique`, wherein an illuminating laser beam is generated and focused onto a pixel defining spot on the surface of an object to be inspected. The illuminated beam is deflected in an oscillatory fashion so as to sweep the spot across the inspected surface. The system is adapted for three different modes of operation. According to the first and second modes, so-called "Transmitted Light Inspection Mode" and "Reflected Light Inspection Mode", the object is point-by-point inspected by means of detecting either transmitted or reflected light, respectively. These modes of operation are timely separated. The third mode of operation, which is aimed at defects classification, is based on detecting both reflected and transmitted beams of light. A single laser beam of incident radiation is directed onto the patterned surface of an object through light deflection means and is either reflected or transmitted, or partly reflected and partly transmitted by the object. This intensity of the incident beam is determined before its interaction with the object. Two separate detectors are accommodated at opposite sides of the object and detect transmitted and reflected beams resulting from this interaction. To this end, the system comprises separate directing optics for receiving the transmitted and reflected beams, respectively, and directing them onto the detectors.
This approach is based on that the interaction of an incident beam with an object to be inspected causes changes in beam's intensity, which changes depend on reflectivity and transmission of the respective region of the object. Hence, by appropriately detecting the intensities of the incident beams and reflected and transmitted beams, respectively, before and after the interaction, each inspectable point, or pixel, on the surface can be represented in a so-called `T-R space`, namely by a point with coordinates corresponding to the transmitted and reflected signal values produced at that point.
However, the system requires very complicated arrangements for illuminating and collecting optics. Indeed, the illuminating arrangement should be provided with the light deflection means and detector appropriately accommodated in the optical path of the incident beam so as to determine the beam intensity prior to the interaction with the object. This complicates and extends the optical path of the incident beam. Moreover, the use of a single beam of incident radiation results in an unavoidable requirement for locating the collecting optics, as well as the detectors for sensing the reflected and transmitted beams, at opposite sides of the object.